Blender Git Loki

Texture Paint: fix artifacts when using masks with symmetry.

For most brushes, texture painting uses a special mask accumulation
table in order to ensure that the amount of added color only increases
when the same pixel is touched multiple times by the stroke.

Unfortunately, only the mask texture was added to the mask before
this check, while normal, stencil, texture alpha masks were applied
after this check. This means that the check can pass if e.g. the
pressure is increased, but the final mask value is actually lower.
One might think that the mask values are fixed per pixel, but with
symmetry that isn't true. The result is a nasty stripe artifact due
to the discrete cutoff nature of the accumulation test.

In order to fix this, apply all masks before accumulation.